07-11-2012, 04:27 PM
Wayne,
Be careful in your understanding of the battery final voltage after a flight. When you measure 3.8v per cell at the end, it may have dropped to 3.5 or 3.4v while you were in last phase of flying with the motor on full throttle. Batteries present their best voltage when there is no load. If you look at the graph below (which is on an LiFePO4 battery so the voltages are slightly lower than LiPo), you will notice where the current draw (red) is high, the voltage (green) drops. Also note than when the load was reduced to zero (motor throttled back), the voltage comes back up again. This final voltage is what you measure at the end of the flight. The motor calcs will always consider the voltage with a motor under powered conditions.
Be careful in your understanding of the battery final voltage after a flight. When you measure 3.8v per cell at the end, it may have dropped to 3.5 or 3.4v while you were in last phase of flying with the motor on full throttle. Batteries present their best voltage when there is no load. If you look at the graph below (which is on an LiFePO4 battery so the voltages are slightly lower than LiPo), you will notice where the current draw (red) is high, the voltage (green) drops. Also note than when the load was reduced to zero (motor throttled back), the voltage comes back up again. This final voltage is what you measure at the end of the flight. The motor calcs will always consider the voltage with a motor under powered conditions.